Presentation Information
[R5-21]Petrological, Mineralogical, and Chemical Comparison of a Type B1 CAI in NWA11343 with Experimentally Synthesized Type B1 CAIs.
*Joris LEONARD1, Yasuaki Tsuruoka1, Shogo Tachibana1 (1. UTokyo Sci.)
Keywords:
Chondrite,CAI,Remelting,Experimental samples,Protoplanetary disk
Calcium-Aluminum-rich Inclusions (CAIs) are the oldest known solids to have formed in the Solar System (4567.30 ± 0.16 Ma; Amelin et al. 2010; Connelly et al. 2012), composed of refractory minerals. Type B CAIs are coarse-grained and are thought to be the result of the melt crystallization of CAI precursors. Type B CAIs are texturally divided into two sub-groups: Type B1 with a pyroxene-rich core and continuous melilite mantle and Type B2 lacking a distinct melilite mantle with homogeneous distribution of phases(e.g., MacPherson 2014). Kamibayashi et al. (2021) experimentally reproduced Type B CAIs in the presence of low-pressure hydrogen gas and demonstrated that a melilite rim of Type B1 CAIs can form through enrichment of Al and Ca near the melt surface through the effective evaporation of Mg and Si. In this study, for better understanding of CAI forming environments through comparison between natural CAIs and experimental analogues, we performed mineralogical and petrographical observations of a Type B CAI (~5 mm in diameter) from a carbonaceous chondrite CV3, NWA 11343 (Gattacceca et al., 2019).
Textural and quantitative elemental analyses of the CAI were performed using a scanning electron microscope (JEOL JCM-7000) equipped with an energy dispersive X-ray spectrometer (JEOL DrySD30) operated at an accelerating voltage of 15 kV. To determine the bulk composition, a manual grid of 861 points (spaced at ~142 µm) was acquired across the inclusion (Tachibana et al. 2003; Tsuruoka et al. under review). The data were divided into four representative subsets to evaluate uncertainties, confirming that the bulk elemental composition of the CAI falls well within the compositional range of Type B CAIs (e.g., Simon & Grossman, 2004).
The core of the CAI is predominantly composed of anorthite (55%), pyroxene (20%), melilite (20%), and spinel (5%), surrounded by a melilite-dominated mantle (melilite 85%) with pyroxene (10%), spinel (3%), anorthite (1%), and traces of metal (<1%). The melilite mantle, ranging from 0.5 to 1 mm in thickness, accounts for approximately 45% of the CAI’s volume, placing the CAI into Type B1. Also, the CAI is surrounded by the Wark-Lovering (WL) rim (1–10 µm in thickness) consisting mainly of diopside. The mantle melilite is enriched in gehlenite at the rim and shows the increase of åkermanite towards the inside, similar to other Type B1 CAIs. This suggests that the melilite mantle began to crystallize from the rim to the interior. We also note that spinel occurs as isolated grains or clusters in the CAI, aligned across a few phases in the core region, and also concentrated between melilite mantle and the WL rim.
Although details will be presented at the meeting, the texture and mineral compositions suggest that this CAI experienced multi-stage melting events before incorporation into the parent body and weak parent body alteration as indicated by the presence of sodalite.
The bulk elemental composition of the CAI shows higher SiO2 content than those of synthetic analogues (Kamibayashi et al. 2021; Tsuruoka et al. under review), which likely causes the difference in the abundances and chemistry of mineral phases: pyroxene is far more abundant and larger in the synthetic samples, while anorthite is more abundant in the CAI. The melilite mantle in the CAI exhibits similar thickness to that of experimental analogues, but the melilite mantle in most of the CAI analogs are more enriched in gehlenite than the CAI mantles. Moreover, the CAI doesn’t contain any grossite, whereas the experimental analogues exhibit grossite beyond the WL rim, most likely formed through evaporation of the melilite mantle.
Textural and quantitative elemental analyses of the CAI were performed using a scanning electron microscope (JEOL JCM-7000) equipped with an energy dispersive X-ray spectrometer (JEOL DrySD30) operated at an accelerating voltage of 15 kV. To determine the bulk composition, a manual grid of 861 points (spaced at ~142 µm) was acquired across the inclusion (Tachibana et al. 2003; Tsuruoka et al. under review). The data were divided into four representative subsets to evaluate uncertainties, confirming that the bulk elemental composition of the CAI falls well within the compositional range of Type B CAIs (e.g., Simon & Grossman, 2004).
The core of the CAI is predominantly composed of anorthite (55%), pyroxene (20%), melilite (20%), and spinel (5%), surrounded by a melilite-dominated mantle (melilite 85%) with pyroxene (10%), spinel (3%), anorthite (1%), and traces of metal (<1%). The melilite mantle, ranging from 0.5 to 1 mm in thickness, accounts for approximately 45% of the CAI’s volume, placing the CAI into Type B1. Also, the CAI is surrounded by the Wark-Lovering (WL) rim (1–10 µm in thickness) consisting mainly of diopside. The mantle melilite is enriched in gehlenite at the rim and shows the increase of åkermanite towards the inside, similar to other Type B1 CAIs. This suggests that the melilite mantle began to crystallize from the rim to the interior. We also note that spinel occurs as isolated grains or clusters in the CAI, aligned across a few phases in the core region, and also concentrated between melilite mantle and the WL rim.
Although details will be presented at the meeting, the texture and mineral compositions suggest that this CAI experienced multi-stage melting events before incorporation into the parent body and weak parent body alteration as indicated by the presence of sodalite.
The bulk elemental composition of the CAI shows higher SiO2 content than those of synthetic analogues (Kamibayashi et al. 2021; Tsuruoka et al. under review), which likely causes the difference in the abundances and chemistry of mineral phases: pyroxene is far more abundant and larger in the synthetic samples, while anorthite is more abundant in the CAI. The melilite mantle in the CAI exhibits similar thickness to that of experimental analogues, but the melilite mantle in most of the CAI analogs are more enriched in gehlenite than the CAI mantles. Moreover, the CAI doesn’t contain any grossite, whereas the experimental analogues exhibit grossite beyond the WL rim, most likely formed through evaporation of the melilite mantle.